Turns out they just counted the citations of the blogger's work.
From Research Recap:
A new study ranks economics bloggers and blogs based on citations to their academic research. The results show a dominant ranking for the University of Chicago’s Becker-Posner blog, with Gary Becker taking the top spot and Richard Posner at number three. In second place is Harvard’s Grew Mankiw. NYU’s Nouriel Roubini is ranked fourth, followed by MIT’s Paul Samuelson. Becker-Posner and Mankiw have a commanding lead over other economics blogs, in terms of scholarly impact.
… it is clear that economics blogging has become a serious endeavor since the advent of the Internet, and some of the nation’s and world’s top economists are engaging audiences worldwide through economics blogs of one form or another.Blogometrics
Franklin G Mixon Jr and Kamal P Upadhyayab